Realization of unconventional physical properties in two-dimensional (2D) transition-metal dichalcogenides (TMDs) is currently one of the key challenges in condensed-matter systems. However, the electronic properties of 2D TMDs remain largely unexplored compared to those of their bulk counterparts. Here, we report the fabrication of a high-quality monolayer NbSe₂ film with a trigonal prismatic structure by molecular beam epitaxy, and the study of its electronic properties by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and electrical transport measurements, together with first-principles band-structure calculations. In addition to a charge density wave (CDW) with 3 × 3 periodicity and superconductivity below 1.5 K, we observed sizable (~0.1 eV) band splitting along the Γ-K cut in the Brillouin zone due to inversion symmetry breaking in the monolayer crystal. This splitting is highly anisotropic in k space, leading to a spin-split van-Hove singularity in the band structure. The present results suggest the importance of spin–orbit coupling and symmetry breaking for unconventional superconductivity and CDW properties in monolayer TMDs.

二维（2D）过渡金属二硫代双金属卤化物（TMDs）中非常规物理性能的实现是目前凝聚态系统中的关键挑战之一。但是，与大批量同类产品相比，二维TMD的电子性能仍未开发。在这里，我们报告了高质量单层NbSe _2的制造分子束外延形成具有三角形棱柱结构的薄膜，并通过扫描隧道显微镜，角分辨光发射光谱和电传输测量以及第一性原理的能带结构计算来研究其电子性能。除了具有3×3周期性和低于1.5 K的超导性的电荷密度波（CDW）外，我们还观察到由于单层晶体中反对称性的破坏，沿布里渊区的Γ-K切口有相当大的（〜0.1 eV）谱带分裂。 。这种分裂在k空间中是高度各向异性的，从而导致能带结构中的自旋分裂van-Hove奇异性。目前的结果表明自旋轨道耦合和对称性打破对于单层TMD中非常规超导性和CDW特性的重要性。